The present invention relates to a process for the manufacture of p-alkylphenols by alkylating phenol with olefins of 3 to 4 carbon atoms, at elevated temperatures, in the liquid phase, in two reaction zones in series and in the presence of strongly acidic ion exchange resins as the catalyst.
It is known to alkylate phenol with olefins in the presence of protonic acids or Lewis acids, such as sulphuric acid or boron trifluoride, respectively. Unfortunately, the use of such catalysts necessitates, for example, the use of corrosion resistant installations. Furthermore, the resulting products cannot be obtained with the necessary purity or with the desired color quality.
Recently, therefore, strongly acidic ion exchange resins in the H-form, especially sulphonated ion exchange resins, such as sulphonated ion exchange resins based on phenol-formaldehyde resins or polystyrene resins, have also been employed as fixed-bed catalysts for alkylation. Although high space-time yields are obtained, local over-heating in the strongly exothermic reaction cannot be reliably excluded. As a result, impure, and, in particular, discolored alkylphenols are formed which are unsuitable for further use. Moreover, the ion exchange resins are damaged.
In the process of DT-OS No. 1,443,346 (equivalent to Australian PS No. 259,106), local overheating is prevented by circulating the reaction mixture, consisting of olefins and phenols and the alkylphenols formed, via a heat exchanger through the reactor and only partially alkylating it. Subsequently, an amount corresponding to the fresh feed of olefins and phenols is withdrawn and the mixture is reacted further in a second stage. A disadvantage of this process is the expenditure required to circulate the reactants and to heat them, which results in decomposition or discoloration of the end products. Moreover, only low space-time yields can be achieved by this process.
According to U.S. Pat. No. 3,257,467, phenols are alkylated with olefins in a single reaction stage, in the presence of strongly acidic ion exchange resins, in a heat-insulated reactor. The catalyst is easily prematurely damaged by local overheating. This process, furthermore, is capable of achieving only space-time yields which are, again, unsatisfactory.
In the process of DT-OS No. 2,526,644, the reaction is carried out using a cation exchanger which has a particle size of 100 to 200 micrometers and is suspended in the liquid reaction mixture. Although, as expected, the removal of the heat of reaction is better with this process, the process is not satisfactory with respect to the yield of the desired p-alkyl compound.
All the processes, however, have the common disadvantage that the alkylphenols formed tend, during the subsequent working-up of the reaction products by distillation, to redissociate into the starting compounds, presumably because of impurities originating from the catalyst. As a result of this, not only is the yield reduced but, especially when readily volatile olefins are formed, such severe vacuum irregularities can be caused in the distillation section that working-up is made very considerably more difficult.